We say a great many things in church (and out of church too) without thinking of what we are saying. For instance, we say in the Creed "I believe in the forgiveness of sins." I had been saying it for several years before I asked myself why it was in the Creed. At first sight it seems hardly worth putting in. "If one is a Christian," I thought, "of course one believes in the forgiveness of sins. It goes without saying." But the people who compiled the Creed apparently thought that this was a part of our belief which we needed to be reminded of every time we went to church. And I have begun to see that, as far as I am concerned, they were right. To believe in the forgiveness of sins is not nearly so easy as I thought. Real belief in it is the sort of thing that very easily slips away if we don't keep on polishing it up.
We believe that God forgives us our sins; but also that He will not do so unless we forgive other people their sins against us. There is no doubt about the second part of this statement. It is in the Lord's Prayer, emphatically stated by our Lord. If you don't forgive you will not be forgiven. No part of His teaching is clearer; and there are no exceptions to it. He doesn't say that we are to forgive other people's sins provided they are not too frightful, or provided there are extenuating circumstances, or anything of that sort. We are to forgive them all, however spiteful, however mean, however often they are repeated. If we don't, we shall be forgiven none of our own."
--The Weight of Glory